df reports wrong size

I had a weird problem the other day when I seemed to be getting inconsistencies between du and df. The two command where in disagreement about what the disk usage was on my box.
thing:/# du -hax –max-depth=1 /
104M total
thing:/# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 3.7G 3.4G 118M 97% /
The first thing I thought was that there was a corruption on the partition table or some other terrible bug. I asked around and got no answers so I went Googling. I came up with nothing although there seemed to be plenty of people with a similar problem.
It then dawned on me what I might have done. I had originally created the Postgres database under /var/lib/postgres under the root file system. As this database got bigger and bigger I had to move it onto its own file system and mount it there. What I had forgot to do was remove the files from the root filesystem after I had confirmed the move was successful. This meant that 2.4Gb of disk space had not been freed on the root file system. Of course when you use du it only adds up the sizes of the files it sees whereas df reports the device usage. So there where no bugs in this case just simple human error.
thingthong:/home/harry# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 3.7G 1.1G 2.6G 31% /